1. Field of the Invention
The present invention relates to the use of Code Division Multiple Access (CDMA) techniques in mobile cellular communication systems and, more particularly, to a method and apparatus for minimizing the delay due to processing of interference cancellation algorithms.
2. Background of the Invention
Most presently available CDMA cellular communication systems use some type of spread spectrum modulation wherein the encoded information signal is spread over a very wide frequency range during transmission. A unique spreading sequence assigned to each user allows multiple users to transmit simultaneously over the same frequency range. The spread spectrum modulated signals may then be despread using the unique spreading sequences to recover the information signals. The information signals are thereafter decoded back into their original form.
Referring to FIG. 1, a detector such as a single-user detector 10 or a multiple-user detector 12, can be used to detect the spread spectrum modulated signal and recover the information signal. In general, the single-user detector 10 detects spread spectrum modulated signals on an individual user basis without relying on information about other users. The multiple-user detector 12, on the other hand, detects spread spectrum modulated signals for several users jointly, i.e., information about one user (e.g., bit decisions, spreading sequences, channel estimates, path profiles, etc.) may also be used for other users.
Referring to FIG. 2, both the single-user and multiple-user types of detectors generally include a conventional receiver 20, a buffer 22, and a de-interleaver/decoder 24. The conventional receiver 20 receives the spread spectrum modulated signal from an antenna assembly 26 and despreads the signal using standard techniques that are well known to those of ordinary skill in the art. The recovered information signal is then passed to the buffer 22 for temporary storage, and subsequently forwarded to the de-interleaver/decoder 24 for de-interleaving and decoding.
The above method, however, does not take into account information about interfering signals that may be present. For example, if too many users are transmitting simultaneously over the same frequency range, a phenomenon called multiple access interference may begin to degrade system performance. To improve the system performance, special receivers called interference cancellation (IC) receivers may be employed. These IC receivers use special interference cancellation algorithms that make use of information about the spread spectrum modulated signal and the interfering signals to optimize the despreading process and enhance recovery of the information signal.
Included among the information used by the interference cancellation algorithms are the spreading sequence mentioned previously and a spreading factor of the spread spectrum modulated signal. The spreading factor, also called the processing gain, is essentially the ratio of the bandwidth of the spread spectrum modulated signal over the bandwidth of the information signal. In general, a higher spreading factor is desirable because the higher the spreading factor, the less susceptible a system will be to interference.
Unfortunately, the complex interference cancellation algorithms introduce a larger delay into the despreading process compared to the conventional receivers. Thus, for some commercial services such as voice services that require short processing delays, use of an advanced interference cancellation algorithm may not be feasible. Moreover, the recently developed third generation standard for CDMA cellular systems requires information about the spreading factor to be distributed over a whole data frame in the information signal. However, the delay of waiting for an entire data frame in order to extract the spreading factor combined with the larger processing delay of the IC receiver may not be acceptable to those services that require short processing delays.
Therefore, it is desirable to be able to minimize any delays incurred in the processing of spread spectrum modulated signals in CDMA cellular systems. More specifically, it is desirable to be able to reduce the processing delay incurred by the interference cancellation algorithms of IC receivers over conventional receivers in such CDMA cellular systems.